Slotted mode decoder state metric initialization

Details Slotted mode decoder state metric initialization Slotted mode decoder state metric initialization

2000-03-31

2003-12-16

Chung, Phung M. (Department: 2133)

Error detection/correction and fault detection/recovery

Pulse or data error handling

Digital data error correction

C375S341000

Reexamination Certificate

active

06665832

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to wireless communications systems. In particular, the invention relates to initialization of a convolutional decoder that has missed a portion of a continuously encoded symbol stream.
BACKGROUND OF THE INVENTION
A wireless communication system may comprise multiple remote units and multiple base stations.
FIG. 1
exemplifies an embodiment of a terrestrial wireless communication system with three remote units
10
A,
10
B and
10
C and two base stations
12
. In
FIG. 1
, the three remote units are shown as a mobile telephone unit installed in a car
10
A, a portable computer remote
10
B, and a fixed location unit
10
C such as might be found in a wireless local loop or meter reading system. Remote units may be any type of communication unit such as, for example, hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment.
FIG. 1
shows a forward link
14
from the base station
12
to the remote units
10
and a reverse link
16
from the remote units
10
to the base stations
12
.
Communication between remote units and base stations, over the wireless channel, can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). An industry standard for CDMA is set forth in the TIA/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny (collectively referred to here as IS-95), the contents of which are incorporated by reference herein in their entirety. Additional information concerning a CDMA communication system is disclosed in U.S. Pat. No. 4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS, (the '307 patent) assigned to the assignee of the present invention and incorporated in its entirety herein by reference.
In the '307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through base stations using CDMA spread spectrum communication signals. The CDMA modulation techniques disclosed in the '307 patent offer many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In a wireless communication system, a signal may travel several distinct propagation paths as it propagates between base stations and remote units. The multipath signal generated by the characteristics of the wireless channel presents a challenge to the communication system. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses changes in time delay, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal received by the remote unit and result in, among other things, fading of the signal. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller in amplitude than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of &THgr; radians, and the second path has an attenuation factor of X dB, a time delay of &dgr; with a phase shift of &THgr;+&pgr; radians, no signal is received at the output of the channel because the two signals, being equal amplitude and opposite phase, cancel each other. Thus, fading may have a severe negative effect on the performance of a wireless communication system.
Typically, modern communication systems use coding to improve immunity to interference and wireless channel noise. Additionally, coding may increase system capacity and improve security. Generally, an information signal is first converted into a form suitable for efficient transmission over the wireless channel. Conversion or modulation of the information signal involves varying a parameter of a carrier wave on the basis of the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At a remote unit, the original message signal is replicated from a version of the modulated carrier received following propagation over the wireless channel. Such replication is generally achieved by using an inverse of the modulation process employed by the base station.
The field of data communications is particularly concerned with optimizing data throughput of a transmission system with a limited signal to noise ratio (SNR). The use of error correcting circuitry, such as encoders and decoders, allows system tradeoffs to be made. For example, smaller SNRs or higher data rates may be used with a particular wireless channel which maintains the same bit error rate (BER).
One class of encoders is known as a convolutional encoder. As is well known in the art, a convolutional encoder converts a sequence of input data bits to a codeword based on a convolution of the input sequence with itself or with another signal. Convolutional encoding of data combined with a convolutional decoder is a well known technique for providing error correction coding and decoding of data. One type of convolutional decoder typically used is a Viterbi decoder.
Coding rate, constraint length, and generating polynomials are used to define a convolutional decoder. A coding rate (k
) corresponds to the number of coding symbols produced (n) for a given number of input bits (k). A constraint length (K) is defined as the length of a shift register used in convolutional encoding of data. Convolutional codes add correlation to an input data sequence by using delay elements (i.e., shift registers) and modulo adders. Taps between the delay elements may terminate at modulo adders forming a desired generating polynomial.
FIG. 2
is a block diagram of a convolutional encoder
20
. The encoder
20
shown contains a shift register
22
tapped at various positions
23
A through
23
N. The shift register taps terminate at one or more of the modulo-2

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